This research explores the etiology of Type 1 Diabetes (T1D) through study of patterns of disease concurrence, especially with other autoimmune diseases (AD). Disease concurrence--the occurrence in an individual of two or more diseases--is a prominent feature of autoimmunity, and of T1D in particular. In preliminary studies we establish the methods to obtain the first true population based disease concurrence prevalence rates, and of quantifying the extent of disease concurrence patterns, long thought to be a characteristic feature of autoimmune disease. The relationships among ADs actually constitute a constellation of interrelated diseases, and are thought to reflect common underlying causative factors, due to the interplay of genetic environmental (including infectious agents), sex-related and early life conditional factors. Clarification of the relative importance of these factors for T1D can be revealed and quantified by the study of AD concurrence. Statistical analysis of the national population-based Swedish health data bases, including two diabetes registries, the hospitalization discharge registry, the birth registry and the cancer registry, will be used to uncover similarities and contrast in patterns of disease concurrence in T1D. Linkage of individuals across registries will permit an assessment of T1D disease concurrence and influential cofactors such as birth weight. By examining the ADs most subject to hospitalization, overall patterns of pairwise disease concurrence will be defined. Both statistically established excess and deficit concurrence patterns will be informative. Immune, endocrine and prenatal events will supply the functional background for interpreting age of onset, order of disease onset, gender and the relationship of infectious disease effects relative to T1D concurrence.